The present invention relates to digital networks, and in particular, to the problem of extending wired networks using wireless links.
The ubiquity of IEEE 802.11 (WiFi) technology makes it a popular way to implement point-to-point wireless links for Ethernet LAN bridging applications. However the aggregate data rate that can be obtained using 802.11a/g endpoints over a single wireless channel is less than 30 Mbps. Even with 802.11n links using 40 MHz channels, the aggregate rate is not likely to be higher than 100 Mbps. Given that wired Ethernet speeds exceed 1000 Mbps, there is a need to close the bandwidth gap between wired and wireless links.
A cost effective way of increasing the data rate on the wireless hop is to bond additional wireless links operating on non-overlapping channels. However, simply adding multiple wireless links to bridge between two Ethernet switches may cause problems:
1. Having more than one link between two LAN switches causes a forwarding loop. Most switches will detect this and shut down the ports connected to the redundant links. This defeats the purpose of adding multiple wireless links.
2. More sophisticated switches implement some form of link aggregation where multiple ports can be configured into a link aggregation group (LAG) and the switch will load balance frames across ports in a LAG. If each port in the LAG is connected to a wireless link endpoint, the switch can potentially send frames over all wireless links concurrently, thus increasing throughput. However, there are several reasons why standard Ethernet link aggregation techniques will not work effectively over wireless links.
1. Changing RF conditions can cause the datarate on a channel to fluctuate, rendering the load balancing algorithm on the switch ineffective. A more agile mechanism that reacts to link conditions in real-time is required.
2. Most link aggregation schemes send control packets on each link to detect link failure. Since wireless links can drop frames due to poor RF conditions, the loss of control frames can cause the switch to falsely detect a link failure leading to poor performance. A failure detection scheme that takes into account the fluctuations in the wireless link is needed.
In summary, aggregating multiple wireless links for Ethernet bridging will not work in a majority of situations where the switches do not support link aggregation. In the cases where switches support link aggregation, the load balancing algorithms do not perform well with the dynamically varying bandwidth of the wireless links.
What is needed is a better way to aggregate multiple wireless links for Ethernet bridging.